How DNA codes for proteins
Gene --> a length of DNA coding for one or more polypeptides
Polypeptide --> a chain of amino acid residues joined by peptide bonds
The genetic code
- Triplet - three bases codes for one amino acid
- Degenerate - all amino acids (except methionine) have more than one code
- Some codes indicate 'stop' - the end of the polypeptide chain
- Widespread but not universal
- A gene unwinds and unzips. Hydrogen bonds break.
- Activated RNA nucleotides bind (complementarily) to the exposed bases. This is catalysed by DNA polymerase.
- Two extra phosphoryl groups are released, giving energy to bond adjacent nucleotides.
- the mRNA produced is complementary to the sequence on the template strand of DNA - it is a copy of the base sequence on the coding strand of DNA.
- the mRNA passes out of the nuclear envelope to a ribosome.
--> The assembly of proteins at ribosomes
Made in the nucleus and passes into the cytoplasm. There are lengrths of RNA folded into hairpin shapes, and has three unpaired bases where an amino acid binds. The other end has three unpaired bases (an anticodon)
1. A molecule of mRNA binds to a ribosome.
2. The tRNA with an anticodon complementary to the first mRNA codon (AUG) forms hydrogen bonds with this codon.
3. The ribosome moves on and the process repeats at the second codon. A peptide bonds forms between the two amino acids
4. The process repeats and the polypeptide chain grows until the stop codon is reached - there are no corresponding tRNAs for the stop codon.
--> A change in the amount of, or arrangement of, the genetic material in a cell
Changes to parts of or whole chromosomes. This may occur during DNA replication, and tabacco, UV light, Xrays and gamma rays may cause mutations
Changes to genes due to changes in nucleotide base sequences.
Point mutation - one base pair replaces another. Also known as substitution. This may be silent (the new codon may give the same amino acid)
Insertion/deletion mutatuons - one or more nucleotide pairs are inserted/deleted. These cause a frameshift - every amino acid after the amino acid is changed.
The Lac Operon
Consists of a regulatory gene (I), the promotor region (P), the operator region (O), and the structural genes X coding for b-galactosidase and Y coding for lactose permease
1. The regulator gene is expressed (transcribed and translated) and the repressor protein is synthesised. It binds to the operator region, covering the part of the promotor region where RNA polymerase normally attaches.
2. The structural genes cannot be expressed. The enzymes are not synthesised.
1. Lactose molecules bind to the other side of the repressor protein, causing the molecule to change shape. It can no longer bind to the operator region, and moves away.
2. RNA polymerase can now bind to the promotor region, initiating the expression of X and Y, and the synthesis of the enzymes.
Lactose permease transports lactose into the cell. B galactosidase catalyses the hydrolysis of lactose
--> Programmed cell death occurring in multicellular organisms.
1. Enzymes break down the cytoskeleton.
2. The cytoplasm becomes dense, with organelles tighly packed. Blebs (lumps on the plasma membrane) form.
3. The chromatin condenses, and the nuclear envelope breaks down. DNA fragments
4. The cell breaks into vesicles that are taken up by phagocytosis.
Apoptosis is an integral part of tissue development.
The chromatin condenses and the chromosomes shorten and thicken. They come together in homogolous pairs to form a bivalent. The non-sister chromatids wrap around each other and attach at points called chiasmata. They may swap sections of cromatids (crossing over). The nuclear envelope disintegrates, and a spindle forms.
Bivalents line up across the equator, randomly assorted.
The homologous chromosomes are pulled by the spindle to the poles. The centromeres do not divide.
Two new nuclear envelopes form, there is a brief interphase, and the chromosomes uncoil.
The nuclear envelope breaks down again. Chromosomes condense and spindles form.
The chromosomes arrange themselves on the equator. They attach to the spindle fibres at the centromeres. Chromatids are randomly assorted.
The centromeres divide, and the chromatids are pulled to opposite poles by the spindle fibres. The chromatids randomly segregate.
Nuclear envelopes reform around the haploid daughter nuclei.
Allele --> An alternative version of a gene
Genotype --> Alleles present within cells of an individual for a particular characteristic
Dominant --> The allele responsible is always expressed in the phenotype
Recessive --> The allele is only expressed in the phenotype if two copies are present
Codominant --> Both alleles contribute to the phenotype
Linkage --> Genes for different characteristics that are present at different loci on the same chromosome are linked.
Sex linkage --> The gene for the characteristic is carried on the X or Y chromosome.
--> The interaction of different gene loci so that one gene locus masks or suppresses the expression of another gene locus.
The second gene (B/b) cannot be expressed if there is no dominant allele (A) present.
A dominant allele (A) at one locus masks the expression of the alleles (B/b) at a second locus.
- Qualitative differences between phenotypes
- Different alleles at a single locus have a large effect on the phenotype
- Different gene loci have quite different effects on the phenotype
- e.g codominance, dominance and recessive patterns of inheritance
- Traits are controlled by 2 or more genes
- Each gene adds to the phenotype
- Different alleles at each locus have a small effect on the phenotype
- Many different polygenic (unlinked) genes can affect the phenotype
Makes the following assumptions:
- The population is large (eliminating sampling error)
- The mating is random
- There is no genotypic selective advantage
- There is no mutation, migration or genetic drift
p^2 + 2pq + q^2 = 1
p= frequency of dominant allele
q=frequency of recessive allele